Albert Barnes Commentary 1 Chronicles 5:9

"and eastward he dwelt even unto the entrance of the wilderness from the river Euphrates, because their cattle were multiplied in the land of Gilead." — 1 Chronicles 5:9 (ASV)

He inhabited - that is, Reuben. Eastward, the Reubenites inhabited the land as far as the commencement of the great Syrian Desert, which extended all the way from the river Euphrates to their borders.